1. Field of the Invention
The present invention relates to a surface acoustic wave device containing an interdigital electrode on a single crystal substrate.
2. Description of the Background
In recent years, mobile communication terminal equipment inclusive of cellular telephones has been rapidly popularized. It is highly desirable to reduce this terminal equipment in size and weight for enhanced portability. To achieve size and weight reductions for the terminal equipment, electronic parts used therewith, too, must be essentially reduced in size and weight. For this reason, surface acoustic wave devices favorable for size and weight reductions, i.e., surface acoustic wave filters, are often used for high- and intermediate-frequency parts of the terminal equipment. A surface acoustic wave device has on the surface of a piezoelectric substrate an interdigital electrode for exciting, receiving, reflecting, and propagating surface acoustic waves.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices, are the surface wave velocity of surface acoustic waves (SAW velocity), the temperature coefficient of a center frequency in the case of filters and of a resonance frequency in the case of resonators (the temperature coefficient of frequency: TCF), and an electromechanical coupling factor (k.sup.2). The characteristics of various piezoelectric substrates currently known for surface acoustic wave devices are set forth below in Table 1.
TABLE 1 __________________________________________________________________________ Propagation SAW Velocity Symbol Composition Cut Angle Direction (m/s) k.sup.2 (%) TCF (ppm/.degree.) __________________________________________________________________________ 128LN LiNbO.sub.3 128.degree.-Rotated Y X 3992 5.5 -74 64LN LiNbO.sub.3 64.degree.-Rotated Y X 4742 11.3 -79 LT112 LiTaO.sub.3 X 112.degree.-Rotated Y 3288 0.64 -18 36LT LiTaO.sub.3 36.degree.-Rotated Y X 4212 4.7 -45 ST Quartz Crystal Quartz Crystal ST X 3158 0.14 0 (first-order coeff.) BGO Bi.sub.12 GeO.sub.20 (100) (011) 1681 1.2 -122 __________________________________________________________________________
As may be seen from Table 1, 64LN and 36LT have an SAW velocity of 4,000 m/s or higher, and are, thus, suitable to construct filters for high-frequency parts of terminal equipment. This is because various systems are practically employed for mobile communications represented by cellular telephones all over the world, and are all used at frequencies of the order of 1 GHz. Accordingly, filters used for high-frequency parts of terminal equipment have a center frequency of approximately 1 GHz. Surface acoustic wave filters have a center frequency substantially proportional to the SAW velocities of piezoelectric substrates used and almost inversely proportional to the widths of electrode fingers formed on the substrates. To enable such filters to be operated at high frequencies, therefore, it is preferable to resort to substrates having high SAW velocities, for instance, 64LN, and 36LT. Also, wide passband widths of 20 MHz or more are required for filters used on high-frequency parts. To achieve such wide passbands, however, it is essentially required that piezoelectric substrates have a large electromechanical coupling factor k.sup.2. For these reasons, much use is made of 64LN, and 36LT.
On the other hand, a frequency band of 70 to 300 MHz is used as an intermediate frequency for mobile terminal equipment. When a filter having a center frequency in this frequency band is constructed with the use of a surface acoustic wave device, the use of the aforesaid 64LN or 36LT as a piezoelectric substrate causes the widths of electrode fingers formed on the substrate to be much larger than those of the aforesaid filter used for a high-frequency part.
This is now explained with reference to roughly calculated values. Here let d represent the width of an electrode finger of a surface acoustic wave transducer that forms a surface acoustic wave filter, f.sub.0 indicate the center frequency of the surface acoustic wave filter, and V denote the SAW velocity of the piezoelectric substrate used. For these values, equation (1) then holds roughly: EQU f.sub.0 =V/(4d) (1)
If a surface acoustic wave filter having a center frequency of 1 GHz is constructed on the assumption that the SAW velocity is 4,000 m/s, then the width of its electrode finger is calculated from equation (1) to be EQU d=4,000(m/s)/(4.times.1,000 (MHz))=1(.mu.m)
On the other hand, when an intermediate-frequency filter having a center frequency of 100 MHz is constructed using this piezoelectric substrate having an SAW velocity of 4,000 m/s, the electrode finger width required for this is given by EQU d=4,000 (m/s)/(4.times.100 (MHz))=10(.mu.m)
Thus, the required electrode finger width is 10 times as large as that for the high-frequency part filter. A large electrode finger width implies that a surface acoustic wave device itself becomes large. To make a surface acoustic wave intermediate-frequency filter small, therefore, it is necessary to use a piezoelectric substrate having a low SAW velocity V, as can be appreciated from equation (1).
Among piezoelectric substrates known to have a very low SAW velocity, there is also ST quartz crystal BGO such as one already referred to in Table 1. A BGO piezoelectric substrate has an SAW velocity of 1,681 m/s. However, the BGO piezoelectric substrate is unsuitable to construct an intermediate-frequency filter for extracting one channel signal alone, because its temperature coefficient of frequency or its TCF is as large as -122 ppm/.degree.C. This is because a large TCF value implies that the center frequency of the surface acoustic wave filter varies largely with temperature. Thus, large TCF is unsuitable for an intermediate-frequency filter because undesired signals may possibly be extracted from other channel adjacent to the desired channel.
Among piezoelectric substrates known to have a relatively low SAW velocity, there is also ST quartz crystal such as one referred to in Table 1. The ST quartz crystal is suitable to construct an intermediate-frequency filter because its temperature coefficient of frequency or its TCF is almost zero (with a first-order temperature coefficient a of zero). For this reason, most of intermediate-frequency surface acoustic wave filters used so far for mobile communication terminal equipment are constructed of ST quartz crystal piezoelectric substrates. However, the SAW velocity of the ST quartz crystal substrate is 3,158 m/s or is not on a sufficiently reduced level, and so presents some limitation to size reductions. In addition, the electromechanical coupling factor k.sup.2 of the ST quartz crystal is 0.14%, and so is relatively small. Small k.sup.2 implies that only a filter having a narrow passband is achievable. Adopted mainly so far for mobile communications, that is, cellular telephones, are analog systems with a very narrow channel width of, for instance, 12.5 kHz according to the Japanese NTT standard, 30 kHz according to the U.S. AMPS standard, and 25 kHz according to the European TACS standard. Thus, the fact that the aforesaid ST quartz crystal has a small electromechanical coupling factor k.sup.2 has offered no problem whatsoever. In recent years, however, digital mobile communication systems have been developed, put to practical use, and so rapidly widespread in view of making effective use of frequency resources, compatibility with digital data communications, etc. The channel width of this digital system is very wide, for instance, 200 kHz and 1.7 MHz in the European cellular telephone GSM and cordless telephone DECT modes, respectively. If ST quartz crystal substrates are used for surface acoustic wave filters, it is then difficult to construct such wideband intermediate-frequency filters using them.
As explained above, a problem with conventional surface acoustic wave devices is that when a piezoelectric substrate having a large electromechanical coupling factor, e.g., 64LN or 36LT is used, it is possible to make its passband wide, but the device size becomes large because that substrate has a high SAW velocity. Another problem occurring is that when the aforesaid BGO substrate having a low SAW velocity is used to achieve device size reductions, adequate selectivity is not obtained due to the large absolute value of the temperature coefficient of frequency, TCF. In either case, adequate characteristics for an intermediate-frequency surface acoustic wave filter are unachievable.
The ST quartz crystal substrate having a small temperature coefficient of frequency or TCF presents some limitation to size reductions due to the fact that its SAW velocity is not sufficiently reduced, and makes it difficult to achieve wide band due to the fact that its electromechanical coupling factor k.sup.2 is relatively small.